I cannot speak from
experience, so keep that in mind

.
I think we can definitely control the succession, it seems to me that is what a food forest is all about. Your alders are early pioneers, in there healing the
land, restoring nitrogen to the soil, covering bare earth where prior forest was cleared out. That is great, you do not have to plant them, they are already there for you.
So you start looking for your next stage in the succession. Bigger, longer lived trees, among others. So, which trees do you want? Pecans and walnuts for nuts and possible timber? Some smaller trees that will fill in and start producing sooner would
be nice, like hazelnuts and various fruit trees, from mulberry and service berry to apples and pears. And you can plan understory plants as well, ramps and
nettles should do well, even with lots of shade. And do not forget to coppice or pollard some of your alders to help
feed those goats of yours.
Geoff Lawton has a
video out where he diagrams the whole process of managing the succession, starting with no trees at all. You have a head start with your alders already there.
As for cutting the trees and turning it into goat pasture, you could, but would that really be best use of the area? And if you wanted it cleared, letting the goats go in and browse might be a good first step, rather than using your resources cutting the trees down.
I would first consider what my overall goals are for the design. Am I working on providing substantial portions of my family food supply? Am I looking at market
gardening for a main source of income? First step, why am I making this design.
Then, how best to use what I have to fulfill the goals. Is the alder patch the natural place to put the food forest, if I want one?
Let us say yes, it is and I do. So now I need to know what my options to grow in my food forest are for my location. Then, which of the choices do my family like, or which choices does the
local market support? Then there are things like
water supply, do I want to do Swales to support my trees? Is there a good
pond location? Before the new trees start going in, any earthworks should be done.
And what to do with the alders as you start introducing your new trees into your forest? Fodder for goats, for some. Load some hugelbeds with trunks for some others. Keep a bunch interspersed with your other trees to keep providing nitrogen, and you can probably coppice those to keep feeding your goats while they keep nursing your other trees.
Pretty sure you do not have to let it go to Douglas Fir.